A SIMPLER FORM OF THE CAMERA OBSCURA.
A simpler form of the camera obscura is seen in Fig. 2. Here the case is a small soap or spice box, the lens a convex spectacle-glass, and the board marked b a partition, serving as a screen upon which the image is thrown. In this form the lens may be fixed in the end of the box if desired, which is much easier than adjusting it in a sliding tube. The focal distance of an ordinary spectacle-glass averages about twelve or fourteen inches, and the box should be, of course, somewhat longer than the focal length of the glass used. A glass from “near-sighted” spectacles will not do, as it is concave instead of convex.
The box is now pointed out of the window, at some well-marked object, such as a sun-lighted building, and the partition-board moved backward and forward, till the point is found at which the image on it is best defined. Then the board may be fastened (as a, b, c, d, Fig. 2) and the top put on, of which the end should be cut off about four inches from the screen, as shown in the figure.
A piece of black cloth thrown over the head, and completely covering the ends of the opening, renders the image more distinct.
THE SKETCHING CAMERA.
